The present invention relates to self-diverting high-rate water packs wherein the gravel is coated with degradable fines that do not substantially mobilize until after the gravel has packed or bridged in a perforation tunnel or fracture.
Traditional operations that place gravel or proppant into subterranean formations use viscosified fluids that are either gelled or gelled and crosslinked in order to increase the fluid's ability to carry suspended particulates. However, in recent years, water packs have become an increasingly popular alternative to conventional gelled slurry gravel packing methods. Water packs are particularly useful in situations wherein the gelled or crosslinked polymers could potentially damage formation permeability. Generally, a water-pack system usually uses non-viscosified brine as the carrier fluid for the gravel or proppant. Rather than relying on viscosity as to carry particulates into perforation tunnels, water packs are used in situations (such as a shorter interval situation) where sufficient velocity can be generated to allow water to carry gravel into the perforation tunnels. Treatments use a blender that can continuously mix gravel and the carrier fluid and supply it to the downhole pump.
More recently, high-rate water packs have come into favor, particularly for use in longer perforated intervals and intervals that are deviated. High-rate water packs are placed at a rate/pressure that is at or near the fracture extension pressure of the reservoir. Placing gravel at such a rate allows for a pressure packing of the perforations and, generally, causes some perforation breakdown. In addition, high rate gravel packs allow for the use of water or a lightly gelled liquid to suspend and carry the gravel based on the flow rate rather than relying on high viscosity.
High-rate water packs are often used to fill perforations, to bypass near-wellbore damage, or to slightly extend fractures while simultaneously filling the fractures with gravel. One known problem in these operations is fluid loss into the formation of the low viscosity carrier fluid once the perforation or fracture has been filled. This is due, at least in part, to the fact that gravel offers relatively little resistance to fluid injection, even when completely filling a perforation tunnel or fracture. Because of the low viscosity nature of the water pack fluid, losses can be severe. Rapid leak off at filled perforations can lead to a risk of a node of gravel building up on the perforation entrance. Such nodes could build to the point of causing a bridge in the screen and screen out the treatment, thus ending the job prematurely. Preferably, once the perforation or fracture is filled, the fluid, and its remaining load of particulates, would divert to fill other perforations and fractures. As used herein “fluid loss” refers to the undesirable migration or loss of fluids (such as the fluid portion of a fracturing or gravel packing fluid) into a subterranean formation.